Electric field controllable birefringence liquid crystal media and optical display devices for use thereof

ABSTRACT

Thin flat panel display devices utilize optical polarization elements to control the amount and location of light transmitted through liquid crystal media having electric field controllable birefringence properties. Nematic liquid crystal compositions are provided, particularly adapted for use in such display systems, that demonstrate electric field controllable birefringence properties, for example, from above room temperature to about 30* Centigrade. These compositions include binary mixtures of ptoluylidene p-n-butylaniline with p-n-butoxybenzylidene p-nbutylaniline or pentyl p-anisylidene p-aminophenyl carbonate with p-ethoxybenzylidene p-n-butylaniline. Ternary compositions may be formed by adding ionizable materials.

United States Patent 1 Soree et a1.

[ 1 ELECTRIC FIELD CONTROLLABLE BIREFRINGENCE LIQUID CRYSTAL MEDIA ANDOPTICAL DISPLAY DEVICES FOR USE THEREOF [75] Inventors: Richard A.Soree, Chestnut Hill;

Mary Jane Rafuse, Harvard, both of Mass.

[73] Assignee: Sperry Rand Corporation, New

York, NY.

[22] Filed: Sept. 9, I971 21 Appl. No.: 178,982

[52] US. Cl. 350/160 LC, 252/408, 23/230 LC 1,170,486 11/1969 GreatBritain [451 Sept. 4, 1973 OTHER PUBLICATIONS V. A. Usol 'Iseva et al.,Chemical Characteristics, Structure, and Properties of Liquid Crystal,Russian Chemical Review, Vol. 32, No. 9, pp. 495-507, (Sept. 1963).

Primary ExaminerGeorge F. Lesmes Assistant ExaminerM. B. WittenbergAttorney-S. C. Yeaton [5 7 ABSTRACT Thin flat panel display devicesutilize optical polarization elements to control the amount and locationof light transmitted through liquid crystal media having electric fieldcontrollable birefringence properties. Nematic liquid crystalcompositions are provided, particularly adapted for use in such displaysystems, that demonstrate electric field controllable birefringenceproperties, for example, from above room temperature to about -30Centigrade. These compositions include binary mixtures of p-toluylidenep-n-butylaniline with p-n-butoxybenzylidene p-n-butylaniline or pentylpanisylidene p-aminophenyl carbonate with pethoxybenzylidenep-n-butylaniline. Ternary compositions may be formed by adding ionizablematerials.

22 Claims, 15 Drawing Figures sum 2 er 5 ISOTROPIC LIQUIID 50- NEMATICLIQUID 20- TEMPERATURE (C) SMECTIC 101 0 2O 4O 6O 80 100 PERCENTP-TOLUYLIDENE P-N BUTYLANILINE FIG.10.

FIG.6.

INVENTORS MAR) JA/VE RAFUSE fi/CHARO A. SOREF 5 ATTORNEY PER CENTTRANSMlSS|ON- PATENTEDQEP 4 PER CENT OPTICAL TRANSMISSlON SHEET 8 0f 5INCIDENT WHITE LIGHT I l I l 8 12 16 2O APPLIED FIELD (VOLTS D.C.)+

FIG.12.

I l -4O '20 O OFF-AXIS ANGLE (DEGREES) INVENTORS MARY JANE RAFUSER/CHARD A. SOREF Pmmsnw' SIIEU 5 BF 5 DYNAMIC SCATTERING FIELD-INDUCEDREALIGNMENT (CONTROLLED BIREFRING m DYNAMIC SCATTERING (9 1 '15 ORE-ALIGNMENT 6'10 I U) HOMEOTROPIC ALIGNMENT o I I I I I I 0.1 1 10 1001k 10k 100k 1M FREQUENCY (HERTZ) I/VVE/VTO/PS FlG.l4.

ATTOR ELECTRIC FIELD CONTROLLABLE BIREFRINGENCE LIQUID CRYSTAL MEDIA ANDOPTICAL DISPLAY DEVICES FOR USE THEREOF BACKGROUND OF THE INVENTION 1.Field of the Invention The invention pertains to improved nematic liquidcrystal compositions and to novel display apparatus including thesecompositions and more especially relates to improved nematic materialsand compositions of materials useful in novel optical instrumentsoperating in the wide temperature range from as low as- Centigrade toabove room temperature.

2. DESCRIPTION OF THE PRIOR ART Certain classes of materials known inthe prior art as nematic liquid crystal materials have been found toexhibit electro-optic effects, these compositions being characterized bytwo transition temperatures. The first is at the transition pointbetween the crystalline solid state and the mesomorphic or liquidcrystal state. The second transition temperature is at the transitionbetween the liquid crystal state and the isotropic fluid state.

It is usually desired to operate optical instruments, includingelectro-optical display devices, at convenient temperatures such as ator near ambient room temperature or at even lower temperatures. Priorart nematic compositions which have solid-to-liquid crystal transitionsabove room temperature must usually by heated to keep them in themesomorphic state. Relatively precise temperature regulation is requiredwith such prior art compositions, which often remain in the liquidcrystal state only over narrow temperature ranges. Instruments such asoptical displays using nematic materials thus often require continuousheating if they are to be ready for instant use.

Nematic liquid crystal materials offer utility, for example, inelectrically controlled display devices of the flat panel type. Forinstance, there are prior art applications of electrically-controllable,dynamic-scattering liquid crystal materials that employ a structurewhich is a cell of sandwhich configuration comprising a transparentplanar front electrode and a specularly reflective back electrodeclosely spaced with respect thereto. Between the two electrodes islocated a thin layer of dynamic light scattering material. With noelectric field applied between the two electrodes, the liquid crystalmaterial is optically transparent. Thus, if a viewer sees a blackbackground specularly reflected in the back electrode, the cell looksblack to a viewer looking into it through its transparent front.However, when a unidirectional electric field is applied between theelectrodes, the liquid abruptly loses its transparent characteristics,scattering any light flowing into it through its transparent frontelectrode. In this state, the scattered light is returned to the viewer,and the apparent color of the cell is generally the same as the lightpassing into it through the front electrode. When the electric field isremoved, the material abruptly reverts to its transparent state andlooks black to the observer.

The scattering effect used in prior art nematic displays in the presenceof an electric field has been explained as being caused by localizedvariations in the effective index of refraction of the medium producedwhen groups of neutral molecules within the medium are set into motionby the electric field. Apparently, ions set in motion through thenormally aligned ne- 2 matic medium supply the initial shearingdisruptive effects. Therefore, some speak of the scattering effect asone produced by the presence of turbulence within the optical medium.

Prior art displays have made advantageous use of the several propertiesof prior liquid crystal dynamic scattering compositions. In one form,these displays have been digital or discrete in nature; a multiplicityof discrete fixed-area electrode segments has been employed, often inregular arrays. Such displays usually embody planar panels with aplurality of discrete electrode segments formed on the display electrodesurface, isolated spatially and electrically from one another.Energization of the display is such that discrete areas of nematicmaterial are either excited or are not excited; i.e., are fully brightin appearance or are dark.

Furthermore, analog displays using dynamic scattering materials areconveniently generated by the apparatus described by R. A. Soref in theUnited States patent application Ser. No. 879,645 for "Liquid CrystalElectro-Optical Measurement and Display Device, filed Nov. 25, 1969,issued as US. Pat. No. 3,675,988 July 11, 1972, and assigned to theSperry Rand Corporation. Soref provides means for producing acontinuously scannable, continuously movable, and continuously alterablebright display image by means of crystalline liquid media controlled tobe transparent or optically scattering by simple control circuitsoperating at relatively low voltage levels. There is provided anelectrically controllable flat screen display by placing a dynamicscattering nematic medium between electrode plates, at least one ofwhich is transparent, the electrode plates forming part of a cellenclosing the nematic medium. The transparent electrode is provided withtwo or more usually different electrical potentials at suitableterminals so that electric field gradients are generated spatiallyacross the display area and also across the thin nematic layer. Aplurality of image configurations may thus be generated by the influenceof the electric fields upon the nematic medium, the images consisting oftranslucent areas. A variety of continuously variable images may begenerated, including large or small area, time-alterable, transparentanalog patterns in a translucent background, or vice versa.

SUMMARY OF THE INVENTION This invention comprises novel liquid crystaldisplay and other optical apparatus employing electrical fieldcontrollable optical birefringence principles and novel electric fieldcontrollable birefringence liquid crystal compositions for employmenttherein. The display apparatus takes the form of a flat cell comprisingtransparentplanar electrodes. Between the two electrodes is placed athin layer of novel electric field sensitive birefringent liquid crystalmaterial. According to the magnitude of the electric field appliedbetween the two electrodes, polarizer elements associated with the cellpermit the flow of light through the cell or prevent such flow. One ofthe novel electric field controllable birefringence materials isrepresented by a family of binary mixtures of electro-optically activecompounds including various proportions of p-n-butoxybenzylidenep-nbutylaniline and p-toluylidene p-n-butylaniline. Another suchcomposition of materials is represented by a family of binary mixturesof electro-optically active compounds including various proportions ofpentyl panisylidene p-aminophenyl carbonate and pethoxybenzylidenep-n-butylaniline. In both cases, advantageous ternary compositions maybe formed by the addition of a small amount of ionizable material, suchas a p-n-alkoxyphenol. These novel electro-optically active liquidcrystal materials are employed in thin layers in optical cells havingtransparent electrodes with means for applying electric fields acrossthe layer. The desired display is formed by selection of appropriatepatterns of electrical fields to be imposed across the active layer, theelectric fields serving to alter the birefn'ngence properties of theliquid crystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in cross section of atransmission form of the invention.

FIG. 2 is a view in cross section of an alternative form of FIG. 1.

FIG. 3 is a cross section view of a reflecting form of the invention.

FIG. 4 is a view in cross section of an alternative form of FIG. 3.

FIG. 5 is a plan view of part of the apparatus of FIG. 1.

FIG. 6 is a view of a display which may be produced by the apparatus ofFIGS. 1 to 5.

FIGS. 7 to 9 are symbolic views useful in explaining operation of theinvention.

FIGS. 10 to are graphs describing the useful properties of the novelcompositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel electro-opticallybirefringent nematic liquid crystal compositions described herein may beemployed, for example, in the unique electrically controllable, flatpanel display devices of FIGS. 1 to 6 for the purpose of generatingdisplays in which the size, Shape, and location of the two-dimensionaldisplay pattern may be changed continuously, as well as in discretesteps. By means of the apparatus of FIGS. 1 to 6, there may be producedcontinuousiy scannable, continuously movable, and continuouslyalterable, bright display images by means of' crystalline liquid mediaexhibiting electrically controlled birefringence phenomena, which mediamay be controlled as to degree of birefringence by simple controlcircuits operating in the novel displays at relatively low voltage andpower levels. Such media are distinct from those normally employed inliquid crystal panel displays, in that they have three particularcharacteristics not employed in media normally used for dynamicscattering or turbulence dis plays; namely, the electrically controlledbirefringent materials herein discussed are characterized:

a. by being homeotropic in a direction perpendicular to the transparentelectrode plates of the cellular panel display when there is no electricfield applied to the electrodes,

b. by having a negative dielectric anisotropy; i.e., e

- e, 0, where a is the dielectric constant parallel to the optical axisof the medium and e, is that perpendicular to the optical axis, and

c. by having the electric field for controlling the display appliedparallel to the optical axis of the medium.

A further distinctive feature of the novel electrooptically active mediaused in the present invention lies in the fact that relatively lowvoltages and low power SUffiCC to operate the display efficiently, sincethe electrically controlled birefringence phenomena employed depends foroperation mainly upon electrical displacement currents, while mediautilizing turbulence or dynamic scattering effects in liquid crystalmaterials require the steady conduction of electrical current throughthe medium. Power dissipation of the order of one microwatt per cm''' ofthe active medium area is readily demonstrated. An additionaldistinctive feature of the novel media of the present invention lies inhaving a sharp turn-on threshold. The spontaneous molecular ordering isunchanged for a range of unidirectional or alternating voltages,starting at zero volts. At a critical threshold voltage of about 9 voltsr.m.s., for example, the ordered pattern is abruptly distorted, thedistortion becoming relatively large within 1 volt r.m.s. above thecritical threshold.

ln FIGS. 1 and 5, a typical construction for the novel display is shown.utilizing a pair of parallel-sided flat glass plates 10 and 11preferably arranged parallel to each other and separated by a thin layer12 of the novel electric field sensitive nematic birefringent liquidcrystal materials according to the present invention. Plate 10 and plate11 are coated on their inner surfaces with thin conducting electrodes 13and 14, respectively. A cell for containing the nematic material isfurther defined by a continuous quadrilateral dielectric wall 15.Extending lineal or elongate voltage terminals 16 and 17 are applied inconductive relation to electrode 14 on glass plate 11 at the'respectiveopposite ends of that electrode. By virtue of their relatively lowresistances, terminals 16 and 17 act as equipotential surfaces. Arelatively small electrical terminal 18 (FIG. 5) may be used inconductive relation with electrode 13 on glass plate 10.

Glass plates 10 and 11 may be made of any suitable glass or of othertransparent insulating materials compatible with the optical and otherrequirements of the cell system. For example, the material may beselected to have an optical index of refraction similar to that of theelectric field sensitive nematic material 12 so as to avoid undesiredreflections at optical interfaces within the cell.

The optically transparent conducting electrodes 13 and 14 may be made ofindium oxide, tin oxide, or other similar materials bonded to glassplates 10 and 1 l by chemical or evaporative deposition, by sputtering,or by other suitable known methods. The choice of materials is such thatconducting electrode 13 has a low resistivity of the order of 50 ohmsper square, for example, so that the whole of electrode 13 may readilyreach the same potential level as is applied to terminal 18. On theother hand, the material of electrode 14 has a relatively highresistivity of about 100,000 ohms per square, for example. Otherresistivity values may be employed, but a relatively high resistivity isbeneficial because ohmic loss within the electrodes is then minimized,thereby preventing appreciable temperature rise in the liquid crystallayer 12. Electrodes l3 and 14 may be interchanged, if desired, or twoopposed electrodes such as electrode 13 may be used. In otherapplications, other configurations of electrodes may be used aselectrodes 13 and 14, including multiple electrodes.

So that the liquid crystal layer 12. may be contained in its pure formand remain protected from contaminants, and be of uniform thickness,dielectric wall 15 is formed as a continuous wall; it is readilyconstructed of sheet material available in the market made of apolymerized fluorocarbon material. The tape is available in thicknessesof the order of 0.25 mils, a thickness suitable for use in the presentinvention. The cell may be held together, at least in part, by aminiscus-shaped film 19 of epoxy or other suitable sealing materialapplied to the external free surface of wall 15 so that it bonds to thatsurface and to the adjacent exterior surfaces of electrodes 13 and 14.

The elongated terminals 16 and 17 on plate 11 and the small terminal 18on plate may be constructed in a conventional manner from anelectricallyconducting silver-epoxy material available on the market orbe deposition of an area of low conductivity tin or indium oxide by oneof the aforementioned processes. As seen in FIG. 5, a voltage source 20for supplying a voltage V13 is connected between terminals 17 and 18,while a second voltage source 21 is connected between the terminals 16and 17 common to electrode 14 for supplying a voltage V14 thereacross.

In operation, a source 30 of collimated white or other light may bearranged to illuminate the face of the cell of FIG. 1, as generallyindicated by the arrow 31, through a linear optical polarizer 32. Theimage formed by electric field activation of the electro-opticallyactive medium 12 is viewed from above plate 10 through a second linearpolarizer or analyzer 33, as by an observer whose eye is placed at 34 inline with the incident light rays. It will be understood that elements30, 32, and 33 are suitably fixed in position in FIG. 1 in relation tothe cell or panel display by support means not shown, and that the sameelements are not shown in FIG. 5, all for the sake of clarity in thedrawings of FIGS. 1 and 5. It will be further understood that thevertical scale of the drawings has been exaggerated, also for the sakeof clarity.

In operation, the apparatus of FIGS. 1 and 5 makes significant use ofthe spatial voltage gradient or variation set up across the transparenthigh resistance electrode means 14. While electrode means 13 may insteadbe used as the high resistance electrode, or both electrodes may be ofhigh resistance material, only the electrode 14 will be considered to bea high resistivity electrode herein for the sake of simplifying thediscussion. with a potential gradient set up across electrode 14, thepotential difference between electrodes 13 and 14 (which is thepotential drop seen across the birefringent liquid crystal layer 12)varies from one spatial location across layer 12 to a next location.This potential variation gives rise to controllable regions oftransparency and opacity within layer 12, providing that the values ofV13 and V14 have been appropriately selected. In the usual mode ofoperation, the polarizers 32 and 33 are crossed. In the regions of layer12 below threshold, the medium is opaque, while in the regions abovethreshold, the medium is optically transmissive. The boundary of thetransisiton region between the transparent and opaque regions isrelatively sharp when employing the novel birefringent liquid crystalmaterials of the present invention.

Referring to FIG. 6, there is seen a typical display 22 producedaccording to the present invention within the novel birefringent liquidcrystal material. The display comprises a rectangular bright area 23 anda rectangular dark area 24 with a common transition boundary 25.Boundary 25 is readily moved to the left or to the right by relativevariation of voltages V13 and V14.

In FIGS. 1, 5, and 6, the rectangular bright area or bar 23 is changedin width by changing the relative magnitudes of voltages V13 and V14according to a predetermined or other pattern. The value of voltage V13may be held fixed, while the value of voltage V14 may be changed, orvice versa. For example, consider the result when voltage V14 is set atzero and voltage V13 is increased from zero. This action causes thebright bar or area 23 to increase in width from zero as boundary 25moves to the right in the drawing, the size of the dark region 24changing correspondingly. Other arrangements which may be modifiedaccording to the teachings of the present invention for producing avariety of similar analog displays are disclosed in the aforementionedUnited States patent application Ser. No. 879,645, such as arrangementswhich create two of the bright movable areas such as area 23 of FIG. 6and which can cause the bright areas to move in cooperative relation soas to expose a movable constant width window or dark area or bar betweenthe two bright areas. Such arrangements may also be used to provide orsimulate indicator elements or pointers by providing variable lengthbars or movable windows to tell a viewer the magnitude of any parameterwhich may be converted into a voltage and used as one of the voltagesV13 or V14. Vertical or horizontal formats are equally possible for thedisplay of temperature, pressure, velocity, acceleration, or otherparameters. A suitable scale may be provided next to the barpresentation, for instance, and values of the parameter involved may beread directly off the scale. The scale may itself be generated byconstant excitation of nematic cells having electrodes shaped or maskedto form numerals. The novel electrically controlled birefringence liquidcrystal compositions may also be used in other display devices,including seven-segment numeric displays and matrix displays and otherdigital displays commonly employing the dynamic scattering effect.

In the arrangement of FIG. 1, it will be understood that when a largearea light source 30 is used, linear polarizer 32 and linear polarizeror analyzer 33 are normally placed substantially in their crossedpositions, extinguishing light in-the entire field of view when there isno electric field applied across medium 12. When an electric field isapplied across medium 12, the observer at 34 will see a highcontrastdisplay at any viewing angle (1) between zero degrees andabout25 degrees off the axis 35 for a typical liquid crystal layer 0.25 milsthick. FIG. 2 is similar to FIG. 1, and similar reference numerals havebeen applied to corresponding elements. The translucent light diffuserscreen 40 added in FIG. 2 between polarization analyzer 33 and theobserver 34 adds to the angular extent of the range of good viewingconditions, although the illumination level supplied by light source 30may need to be increased.

The invention may be operated in the transmissive mode, as in FIGS. 1and 2, or may be operated in a reflective mode, as in FIGS. 3 and 4. InFIG. 3, the cell is similar to that of FIGS. 1, 2, and 4, but theelectrode 14 is made relatively thicker, having an inner surface 45 ofchromium plate or of other smooth metallic material polished so thatsubstantially all light from source 30 is reflected, traveling twicethrough medium 12 before reaching the observer's eye at 34. In addition,the cell of FIG. 3 is equipped externally with a circular polarizer 41.Again, as in FIGS. 1 and 2, the liquid crystal medium 12 is homeotropicin the absence of an electric field across it, behaving as an isotropicmaterial for light propagating perpendicular to the display. Lightentering the medium 12 of FIG. 3 is, for example, circularly polarizedin a clock-wise sense and undergoes no net phase retardation inpropagating through medium I2. When reflected off of mirror surface 45,the light becomes circularly polarized in the opposite sense and issubstantially totally absorbed by circular polarizer 41 upon re-enteringit. Hence, the total area of the display appears dark to the observerseye at location 34, for example, when no electric field appears acrossmedium 12.

When a predetermined electric field is placed across the liquid crystalmedium 12, the display of FIG. 6 is again produced, because theeffective birefringence of the liquid crystal medium 12 becomes large.Such results in a change of the ellipticity of light traversing themedium; hence, the display appearance changes from dark to lightwherever the electric field across the medium is sufficient.

The cell in the reflection mode display of FIG. 4 is like the cells ofFIGS. 1 and 2 in that light can pass entirely through the cell. Thecircular polarizer 41 of FIG. 3 is again used, but the interiorreflecting surface 45 is placed outside of the cell of FIG. 4 in theform of mirror 43. The operation of the device is otherwise the same asthe operation of the reflective mode device of FIG. 3. If desired, alight diffusing screen 42, such as may be constructed of opal glass, maybe interposed between plate 11 and mirror 43 so as to broaden theangular viewing range (1) of the device.

As has been observed in the foregoing, the novel display of the presentinvention is particularly concerned with the beneficial use of certaintypes of liquid crystal materials that exhibit electrically controllableoptical birefringence properties, rather than simply with conventionalturbulence or dynamic scattering characteristics. Examples of suchelectrically controllable birefringent materials remain to be discussedherein.

Materials of the electrically variable birefringent type have propertiesthat may, in a schematic way, be represented as in the three views ofFIGS. 7 to 9 according to the electric fields action upon the molecularorientation within the film. A side view like FIG. 1 of the opticallyactive cell is illustrated and the thickness of layer 12 is againgrossly exaggerated. The small bars, such as bar 50, indicate the localordering directions of molecules, while the arrows, such as arrow 51,indicate the directions of molecular polarization, each polarizationvector representing a vector summation of the permanent dipole momentand the induced polarization of the molecule; The ordering when E,produces an equilibrium molecular pattern, imposed by electrostaticattraction or wall forces (originating with the walls of electrodes 13and 14) and by intermolecular forces. The perpendicular alignment ispromoted by cleaning the electrode walls with a sulfuric acidbichromatesolution, by coating the walls with a surfactant, and by avoidingrubbing the wall surfaces during the cleaning process. The homeotropicalignment tends to suppress the emergence of dynamic scattering in thelayer for unidirectional voltages in the range from zero to about volts,and for alternating voltages from zero to about 14 r.m.s. As abovenoted. films of 0.25 mils or less thickness also aid in increasing thefield of view of the display.

If the electric field E of FIG. 7 is zero or rather weak, spontaneousordering prevails and the birefringence of medium 12 is substantiallyzero for light propagating substantially normal to medium 12 (along theoptical axis of the medium). Due to the negative anisotropy of medium12, the angle between the axis and the molecular polarization is 45 andThe applied electric field exerts a dielectric torque on the molecularpolarization, tending to rotate it into parallelism with the field. Whenthe field I1" becomes sufficiently high, as in FIG. 8, it may partiallyovercome the elastic restoring forces of the spontaneous pattern presentwhen the field E is weak or zero; then, the molecules rotatecollectively into the new configuration of FIG. 8. A further increase inelectric field strength, as represented by E in FIG. 9, produces furtherrotation, with a rotation near 90 being possible. When the field E isdropped to zero, the molecules relax, again taking up the relativepositions of FIG. '7.

The molecules of liquid crystal materials suitable for the purpose arenot, in general, cylindrically symmetric about their long axes and therewill therefore be some three-dimensional randomness in the orientationof the short molecular axes in the original zero-field case, asrepresented only in two dimensions in FIG. 7. Thus, when re-ordering iscommanded by the imposition of a finite electric field, the long axes ofthe molecules will take up locations substantially within a cone havingan axis centered on the electric field direction. This feature causesthe optical-axis orientation of individual clusters of molecules tochange from one region to the next as seen in FIG. 9, for example,although each molecular cluster has the same birefringence.

The electric field re-ordered nematic material of FIG. 9 no longerappears entirely isotropic to normally incident light. The opticalindicatrix of the medium has been distorted significantly and the mediumhas become birefringent along the axis of the electric field vector Efor example. Orthogonal linearly polarized light rays now travel withdifferent speeds through the optically active medium 12.

Referring again to the light transmission forms of the invention shownin FIGS. 1 and 2, it is seen that these devices may be operated as lightvalves. In the transmissive mode, the cell is placed between the linearpolarizer 32 and the linear analyzer 33; one useful procedure is to havethe polarizers 32 and 33 in crossed relation, extinguishing lighttransmission when voltage E is zero or below the characteristicthreshold. If E is raised above the threshold value to E for instance,birefringence of a part or of all of the medium 12 is increased andoptical transmission correspondingly increases over a portion or all ofthe area 22 of FIG. 6. It will be seen that other analog or digitaldisplays may be operated in a similar manner.

For example, in the reflective analog displays of FIGS. 3 and 4,circular polarizer 41 is required on the optical input side of the cell.The circular polarizer 41 consists in the conventional manner of alinear polarizer and a quarterwave plate; the linear polarizer produceslinearly polarized light with orthogonal components a and B forinjection into the quarter-wave plate. On its first trip through thequarter-wave plate, the 5 component may suffer a 90 phase retardationrelative to the a component. When the electrically controlledbirefringent medium 12 is not exposed to a field above its thresholdvalue, no additional relative phase shift is produced. Mirror surface 45also treats the phases of the a and B components equally. Afterreflection of the light components at surface 45, and after the secondtrip through the quarter-wave plate of circular polarizer 41, the Bcomponent experiences an additional 90 relative phase retardation.Therefore, the reflected light returns to the linear polarizer of device41 and is substantially totally absorbed thereby. Above the electricfield threshold value, as at the increased field strength E the lightreturning through circular polarizer 41 is elliptically polarized overat least part of the field of view, and the light passes to the observerat 34.

One kind of multiple-component electrically controllable birefringenceliquid crystal composition described herein includes as a commoncomponent a p-nalkoxybenzylidine p-n-butylaniline, where the alkoxyradical may be an ethoxy or butoxy radical, and which may be identifiedby the general formula:

where R for the ethoxy material is C l-l or, for a butoxy material, is C11 The constituent p-n-butoxybenzylidene p-nbutylaniline may begenerated according to the following procedure. A mixture ofcommercially available pn-butylaniline (1.5 grams, 0.1 mole) and ofcommercially available p-n-butoxybenzaldehyde (1.8 grams, 0.01 mole) wasrefluxed for two hours in ethanol. A reaction product resulted which,after filtration and recrystallization, provided 2.3 grams (a 75 percent yield) of p-n-butoxybenzylidene p-n-butylaniline. This materialdemonstrated a melting point of +24 Centrigrade with a smectic phasefrom +24 to +45 Centigrade and a nematic phase from +45 to +7 6Centigrade. The infrared spectrum in CCl showed the presence of no H-N,no H-CO, and no HC=O bands.

A second component of the one kind of multiplecomponentelectro-optically active crystal composition is p-toluylidenep-n-butylaniline; this novel material was first described and claimed inthe U.S. Pat. application Ser. No. 128,666 to M. J. Rafuse entitled:Liquid Crystal Composition and Devices", filed Mar. 29, 1971, issuedJuly 11, 1972 as U.S. Pat. No. 3,675,987,

identified by the formula:

As described in the patent application Ser. No. 128,666, the novelcompound p-toluylidene p-nbutylaniline may be generated by the followingmethod. Commercially available p-tolualdehyde d(24.03 grams, 0.2 mole)and commercially available p-n-butylaniline (29.85 grams, 0.2 mole) wererefluxed for 2 hours in 25 ml of absolute ethanol. The solvent wasremoved on a rotary evaporator and the residue was distilled three timesunder reduced pressure to yield 34.59 grams of a constant boilingfraction (b.p. +1 39 to +141 Centigrade at 0.06 mm of mercury) ofp-toluylidene p-n-butylaniline (a 69 per cent yield). The refractiveindex of the pale yellow liquid was 1.6085 at +24 Centigrade. Theinfrared spectrum in CCl, showed the presence of no N-H, no H-CO, and noHC=O bands.

A wide range of relative precentages of the constituent materialsp-n-butoxybenzylidene p-n-butylaniline and p-toluylidenep-n-butylaniline has been found useful, and mixtures in this range ofthe two-component composition may be prepared generally in the mannerused for preparing a mixture of per cent by weight ofp-n-butoxybenzylidene p-n-butylaniline and 30 per cent by weight ofp-toluylidene p-n-butylaniline. The proper proportions of the twoconstituents are melted together to form the isotropic phase of themixture and then are cooled with continued stirring. The particular70-30 per cent mixture exists in the nematic phase from +l4 to +56Centigrade and in the smectic phase from 35 to +14 Centigrade.

The useful range of relative weight percentages discovered for themixture of p-n-butoxybenzylidene p-nbutylaniline with p-toluylidenep-n-butylaniline is shown in FIG. 10, with the circles representingexperimentally derived points. The line 100, representing the boundarybetween isotropic and nematic liquid states, is seen to slopesignificantly from nearly +60 Centigrade to below +20 Centigrade. Theline 101, representing the boundary between nematic and smectic states,falls from about +15 Centigrade to about 35 Centigrade with a similarslope. Dotted lines coupled to the boundary lines and 101 representreasonable extrapolation of the experimentally determined curves. As isseen from FIG. 10, the novel mixture of materials presents a wide rangeof choices of actual compositions, all of which have useful nematiccharacteristics at or below the usual ambient temperature.

The p-toluylidene p-n-butylaniline has the favorable action of greatlyreducing the operating temperature of the composition in which it isemployed. The temperature depressant material has the general geometricsymmetry of the kind which often characterizes molecules having liquidcrystal characteristics. Howver, it has no dipole characteristics atright angles to the long axis of the molecule. Therefore, itsintermolecular interactions are too weak to confer liquid crystalproperties upon it. Because of its long, rod-like shape, theptoluylidene p-n-butylaniline molecule will fit compatibly between othermolecules actually having good liq uid crystal properties, weakeningsuch intermolecular attraction and consequently lowering the operatingtemperature range of the multi-cornponent composition.

Relatively small additive proportions of easily ionizable materials maybe added to the foregoing binary material, such as substantially one percent by weight of an ionizer such as p-n-ethoxyphenol orp-nbutoxyphenol. The effect of the ionizer material is noted, forexample, when the material is used in turbulence displays by asignificant increase in the turbulence effect and in the life of thedisplay. The effect of the ionizer is particularly felt, in the casewhere the medium is used as a controllable birefringence medium, inbeneficially decreasing the realignment threshold.

A second novel type of composition of optically active materials usefulin the present invention includes mixtures of pentyl p-anisylidenep-aminophenyl carbonate and an alkoxybenzylidene material such aspethoxybenzylidene p-n-butylaniline, which mixtures have nematic phasesfrom below -20 to +70 Centigrade.

The p-ethoxybenzylidene p-n-butylaniline, which may be represented bythe formula:

may be made as follows. Commercially available pethoxybenzaldehyde (7.5grams, 0.05 mole) and p-nbutylaniline (7.5 grams, 0.05 mole) wererefluxed in 20 ml of dry ethanol for 3 hours. The material was thencooled, filtered, and recrystallized three times from ethanol to yield8.12 grams of the white material pethoxybenzylidene p-n-butylanilinewith a melting point (nematic) from +32 to +77 Centigrade. This was a 58per cent yield. The infrared spectrum in CCl. showed the presence of noN-H and no C=O bands. The making of pentyl p-anisylidene p-aminophenylcarbonate, which may be identified by the formula:

is a more complex procedure involving first making as an intermediateproduct the compound panisylidene p-aminophenol, which may be recognizedby the for- The compound p-anisylidene p-aminophenol may be madeaccording to the following steps. Commercially available p-anisylidene(6.8 grams, 0.05 mole) and paminophenol (5.5 grams, 0.05 mole) wererefluxed in 25 ml of dry ethanol for 2 hours. Cooling, filtering, andrecrystallizing from ethanol gave 9.64 grams, which was an 85 per centyield, of p-anisylidene paminophenol. The material was off-white incolor with a melting point lying between +l89 and +l90 Centigrade. Theinfrared spectrum in mineral oil showed a broad response to the OHvibration at 3200 to 2500 cm".

For completing the second novel mixture of materials, p-anisylidenep-aminophenol (4.6 grams, 0.02 mole) was put in 20 ml of dry pyridineunder nitrogen and 4.2 grams of triethylamine was added. To the mixturewas added, drop by drop. 6.6 grams (0.02 mole) of'amyl chloroformatedissolved in ml of benzene. After this event, the reaction mixture wasstirred for hours, then flooded with water and extracted three timeswith 100 ml of benzene. The benzene layer was washed with water, dried,and evaporated to yield an oil-like material that solidified at +3Centigrade. Recrystallization from ethanol, from hexane, and then againfrom ethanol gave 3.54 grams (a 52 per cent yield) of white pentylp-anisylidene p-aminophenyl carbonate with a melting point (nematic)lying between +46 and +80 Centigrade. The infrared analysis in CCl,showed no O-H or NH or OCH bands. The presence of the C=0 band wasindicated at 1760 cm I A wide range of relative percentages of theconstituent materials p-ethoxybenzylidene p-n-butylaniline and pentylp-anisylidene p-aminophenyl carbonate has been found to be useful, andmixtures in this range of the two-component composition may be preparedgenerally in the same manner as is used in preparing a typical mixtureof 25 per cent pentyl p-anisylidene paminophenyl carbonate and per centpethoxybenzylidene pn-butylaniline by weight. The proper proportions ofthe two constituents are melted together to form the isotropic phase ofthe mixture and then are cooled while stirring.

For example, a mixture of 25 per cent by weight of pentyl p-anisylidenep-aminophenyl carbonate and 75 per cent by weight p-ethoxybenzylidenep-n butylaniline was prepared by heating the two materials together toabove the isotropic transition temperature and the melt was mixed byagitation while cooling to the nematic phase. This mixture exhibited anematic liquid phase from below 20 to +77 Centigrade.

The useful range of relative weight percentage discovered for mixturesof p-ethoxybenzylidene p-nbutylaniline and pentyl panisylidenep-aminophenyl carbonate is illustrated in FIG. ll, with the circlesrepresenting experimentally derived points. The line 150, representingthe boundary between isotropic and nematic liquid phases, is seen toextend at about +80 Centigrade, but with a slight slope. The line 1511,representing the boundary between the nematic liquid and the solid statefalls rapidly from about +27 Centigrade to 25 Centigrade then, in ashort interval, falls below -30 Centigrade before climbing again tohigher temperatures. The dotted lines illustrate reasonableextrapolations of the experimentally determined curves.

As is clear from observation of FIG. 11, the second novel mixture ofmaterials offers a wide range of choices of actual compositions all ofwhich have valuable nematic liquid crystal characteristics at or belowusual ambient temperatures. Whether the material is used in abirefringence display or in a turbulence display small amounts ofionizer materials may readily be added, such as p-n-ethoxyphenol orp-n-butoxyphenol.

The properties of the two novel multi-component, electricallycontrollable birefringence compositions of materials are generallysimilar and may be illustrated for the sake of brevity by presentinginformation on the properties of the binary composition having 25 percent by weight of pentyl p-anisylidene p-aminophenyl carbonate and 75per cent by weight of pethoxybenzylidene p-n-butylaniline when employedin a display cell such as shown in FIG. 1, for example. Theelectro-optical transfer characteristic of FIG. 12 is the measuredcharacteristic for white light with a constantdirection, steady-stateelectric field across a 0.5 mil layer of the medium 12 at roomtemperature. The collimated white light was normally incident withpolarizers 32 and 33 crossed. it is seen that the transparency of thelight valve jumps abruptly from a normalized value of l per cent at 8.2volts to 55 per cent at 8.8 volts. If the medium 12 is exposed to analternating electric field, the transition occurs at a lower r.m.s.voltage. For example, the percentage optical transmission in theselected example jumps abruptly from a normalized value of l per cent at5.8 volts r.m.s. to a normalized value of 80 per cent at 6.5 voltsr.m.s.

The two principal multi-component electrically controllablebirefringence compositions may also be employed in dynamic scatteringdisplays of the analog and digital types since, under certainpredetermined conditions, the materials may also present electric fieldcontrollable turbulence effects. Dynamic scattering may be induced inboth principal compositions at relatively low audio frequencies.However, the appearance of dynamic scattering effects is readilycontrollable, since the dynamic scattering in the two compositions isrelatively weak and its threshold occurs at a unidirectional field of 16to 20 volts, which is well above the threshold for induced dielectricre-alignment. Thus, it is possible to use the novel materials in oneoptically active mode or the other with complete absence of overlappingor interfering effects.

FIG. 13 illustrates the several regimes which appear in the selectedexample of per cent pentyl panisylidene p-aminophenyl carbonate and 75per cent p-ethoxybenzylidene p-n-butylaniline composition, for example.The graph of the figure illustrates the observed frequency-dependence ofboth thresholds, showing the distinct homeotropic alignment, dynamicscattering, and field induced birefringent realignment patterns andtheir mutual boundaries. In the figure, the realignment threshold isdefined as the r.m.s. voltage at which the optical transmission firstreaches 50 per cent of its maximum. Similarly, the dynamic scatteringthreshold is the r.m.s. voltage at which the timeaveraged, on-axispolarized light transmission drops 50 per cent from its zero-voltagevalue. The birefringent response is seen to extend from zero to 350,000cycles per second.

FIG. 14 was derived in the same way as FIG. 13, but it illustrates thesituation in which an ionizable aromatic compound is added to thenematic liquid used in FIG. 13. The birefringence realignment voltage isfavorably reduced. Where the novel composition is employed in thedynamic scattering mode, the maximum usable frequency for achievingdynamic scattering is increased. In deriving FIG. 14, 1 per cent byweight of p-n-butoxyphenol was added to the liquid medium used in FIG.13, thus increasing the electrical conductivity of the liquid medium bya factor of 40. It is seen that the upper frequency for dynamicscattering is increased from 20 to 400 cycles per second. Also, themidfrequency realignment threshold is favorably decreased from 6.6 voltsr.m.s. to 5.5 volts r.m.s.

Angular width of view typical of the novel compositions is representedin FIG. 15, where optical transmission for two different thicknesses ofthe medium 12 of FIG. 1 is plotted as a functionof the viewing angle 4)with respect to the normal (for 0.25 and for 0.5 mil thick media). Theangular field of view was obtained by plotting transmitted lightintensity as a function of angle (1: measurements being'made in the zerofield state and with a unidirectional field of 10 volts across themedium 12. It is seen that the optical contrast ratio between the twostates may be greater than 100 to 1 within a 20 degree cone.

The versatile nature of the novel display and electrically controllablebirefringence liquid crystal media of the present invention is clearfrom the foregoing. Readily adaptable to-use in analog and digital typesof liquid crystal panel displays, the novel features of the inventionmay also find use both in transmissive and in reflective types of suchdisplays. Low driving direct or alternating voltages may be employed,voltages that are considerably lower than those required for dynamicscattering display devices. Since the novel effect is largely anelectric field polarization effect, the currents used are primarilydisplacement currents and operating power is consequently lower than indynamic scattering displays, where operation depends upon the transportof mobile charge carriers. Enhanced life times of operation may bedemonstrated, such as on the order of several thousand hours, sinceelectrolytic and other degrading effects due to conduction currents areabsent. While the brightness of the controlled birefringence display isslightly lower than that of the dynamic scattering or turbulencedisplay, contrast is greater and analog bar-graph displays, for example,show very sharp boundaries between light and dark portions of thedisplay. Prior art liquid crystal materials have generally not beennematic at room temperature and have relatively high voltage thresholdsas compared to the present materials. Also of material significance inpermitting use of low cost driver circuits is the fact that prior artliquid crystal media are generally transparent in the zero voltage stateand are opaque above the threshold voltage. The reverse is true of thenovel liquid crystal media, making design of the effective drivercircuits more convenient.

According to the invention, there are provided room temperatureelectrically controllable birefringence liquid crystal compositions ofmatter particularly suitable for use as nematic liquid crystalcompositions in the novel optical display or in optical switches orother optical instruments. Binary and ternary compositions are discloseduseful in such instruments at temperatures between 30 and +70Centigrade, whereas few prior art liquid crystal compositions displayuseful properties below +20 Centigrade.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than of limitation and that changes within thepurview of the appended claims may be made without departure from thetrue scope and spirit of the invention in its broader aspects.

We claim:

1. An electro-optically active composition of materials consisting of:

an alkoxybenzylidene p-n-butylaniline, and

pentyl p-anisylidene p-aminophenyl carbonate.

2. The composition described in claim 1 wherein the alkoxy radical isp-ethoxy.

3. The composition described in claim 1 wherein the alkoxy radical isp-butoxy.

4. The composition described in claim 1 consisting of:

an alkoxybenzylidene p-n-butylaniline as represented by the formula:

pentyl p-anisylidene p-aminophenyl carbonate as represented by theformula:

5. The composition described in claim 1 containing substantially 20 toparts in a hundred parts by weight of the compound pentyl p-anisylidenepaminophenyl carbonate, a substantial portion of the remainder being thecompound p-ethoxybenzylidene p-nbutylaniline.

6. The composition described in claim 1 containing substantially 20 to40 parts in a hundred parts by weight of the compound pentyl panisylidene paminophenyl carbonate, a substantial portion of theremainder being the compound p-ethoxybenzylidene p-nbutylaniline.

7. The composition described in claim 1 containing substantially 22 to40 parts in a hundred parts by weight of the compound pentylp-anisylidene paminophenyl carbonate, a substantial portion of theremainder being the compound p-ethoxybenzylidene p-nbutylaniline.

8. The composition described in claim 1 containing substantially 27.5parts by weight of the compound pentyl p-anisylidene p-aminophenylcarbonate and substantially 72.5 parts by weight of the compoundpethoxybenzylidene p-n-butylaniline.

9. The composition described in claim 2 to which is added substantiallyone per cent by weight of a pmalkoxyphenol.

10. An electro-optically active composition of materials consisting of:

an alkoxybenzylidene p-n-butylaniline, and

p-toluylidene p-n-butylaniline.

11. The composition described in claim 10 wherein the alkoxy radical isp-n-butoxy.

12. The composition described in claim 10 consisting of:

p-n-butoxybenzylidene p-n-butylaniline as represented by the formula:

p-toluylidene p-n-butylaniline as represented by the formula:

tion of the remainder consisting of the compound p-nbutoxybenzylidenep-n-butylaniline.

14. The composition described in claim 10 consisting of substantially 36to 82 per cent by weight of the compound p-toluylidene p-n-butylaniline,a substantial portion of the remainder consisting of the compound p-nbutoxybenzylidene p-n-butylaniline.

15. The composition described in claim 10 consisting of substantially 42to 82 per cent by weight of the com pound p-toluylidenep-n-butylaniline, a substantial portion of the remainder consisting ofthe compound p-nbutoxy benzylidene p-n-butylaniline.

16. The composition described in claim 10 to which is addedsubstantially l per cent by weight of a p-nalkoxyphenolv 17. Anelectro-optically active device comprising:

container means for supporting a layer of electrooptically activematerial, and

transparent electrode means constituting a portion of said containermeans for applying an electric field pattern across said active layer,said layer consisting of:

an alkoxybenzylidene p-n-butylaniline, and pentyl p-anisylidenep-aminophenyl carbonate.

18. Apparatus as described in claim 17 wherein said layer includesp-ethoxybenzylidene p-n-butylaniline.

19. Apparatus as described in claim 18 wherein said layer additionallyincludes a p-n-alkoxyphenol.

20. An electro-optically active device comprising:

container means for supporting a layer of electrooptically activematerial, and

transparent electrode means constituting a portion of said containermeans for applying an electric field patternacross said active layer,said layer consisting of:

an alkoxybenzylidene p-n-butylaniline, and ptoluylidenep-n-butylaniline.

21. The apparatus described in claim 20 wherein said layer includesp-n-butoxybenzylidene p-n-butylaniline.

22. The apparatus described in claim 21 wherein said layer additionallyincludes a p-n-alkoxyphenol.

I t i t k

2. The composition described in claim 1 wherein the alkoxy radical isp-ethoxy.
 3. The composition described in claim 1 wherein the alkoxyradical is p-butoxy.
 4. The composition described in claim 1 consistingof: an alkoxybenzylidene p-n-butylaniline as represented by the formula:5. The composition described in claim 1 containing substantially 20 to65 parts in a hundred parts by weight of the compound pentylp-anisylidene p-aminophenyl carbonate, a substantial portion of theremainder being the compound p-ethoxybenzylidene p-n-butylaniline. 6.The composition described in claim 1 containing substantially 20 to 40parts in a hundred parts by weight of the compound pentyl p-anisylidenep-aminophenyl carbonate, a substantial portion of the remainder beingthe compound p-ethoxybenzylidene p-n-butylaniline.
 7. The compositiondescribed in claim 1 containing substantially 22 to 40 parts in ahundred parts by weight of the compound pentyl p-anisylidenep-aminophenyl carbonate, a substantial portion of the remainder beingthe compound p-ethoxybenzylidene p-n-butylaniline.
 8. The compositiondescribed in claim 1 containing substantially 27.5 parts by weight ofthe compound pentyl p-anisylidene p-aminophenyl carbonate andsubstantially 72.5 parts by weight of the compound p-ethoxybenzylidenep-n-butylaniline.
 9. The composition described in claim 2 to which isadded substantially one per cent by weight of a p-n-alkoxyphenol.
 10. Anelectro-optically active composition of materials consisting of: analkoxybenzylidene p-n-butylaniline, and p-toluylidene p-n-butylaniline.11. The composition described in claim 10 wherein the alkoxy radical isp-n-butoxy.
 12. The composition described in claim 10 consisting of:p-n-butoxybenzylidene p-n-butylaniline as represented by the formula:13. The composition described in claim 10 consisting of substantially 20to 72 per cent by weight of the compound p-toluylidene p-n-butylaniline,a substantial portion of the remainder consisting of the compoundp-n-butoxybenzylidene p-n-butylaniline.
 14. The composition described inclaim 10 consisting of substantially 36 to 82 per cent by weight of thecompound p-toluylidene p-n-butylaniline, a substantial portion of theremainder consisting of the compound p-n-butoxybenzylidenep-n-butylaniline.
 15. The composition described in claim 10 consistingof substantially 42 to 82 per cent by weight of the compoundp-toluylidene p-n-butylaniline, A substantial portion of the remainderconsisting of the compound p-n-butoxy benzylidene p-n-butylaniline. 16.The composition described in claim 10 to which is added substantially 1per cent by weight of a p-n-alkoxyphenol.
 17. An electro-opticallyactive device comprising: container means for supporting a layer ofelectro-optically active material, and transparent electrode meansconstituting a portion of said container means for applying an electricfield pattern across said active layer, said layer consisting of: analkoxybenzylidene p-n-butylaniline, and pentyl p-anisylidenep-aminophenyl carbonate.
 18. Apparatus as described in claim 17 whereinsaid layer includes p-ethoxybenzylidene p-n-butylaniline.
 19. Apparatusas described in claim 18 wherein said layer additionally includes ap-n-alkoxyphenol.
 20. An electro-optically active device comprising:container means for supporting a layer of electro-optically activematerial, and transparent electrode means constituting a portion of saidcontainer means for applying an electric field pattern across saidactive layer, said layer consisting of: an alkoxybenzylidenep-n-butylaniline, and p-toluylidene p-n-butylaniline.
 21. The apparatusdescribed in claim 20 wherein said layer includes p-n-butoxybenzylidenep-n-butylaniline.
 22. The apparatus described in claim 21 wherein saidlayer additionally includes a p-n-alkoxyphenol.